1) Yes! There's a demo of a network game at
the project's page, and same-computer multiplayer can be done with scripting (check out
this topic).
2) Yep. I've seen it done in
Samurai Blade.
3) Message boxes are easy. A sequence of them (conversation-style) is easy. Multiple responses... may be a little tricky. I can't think of an easy way off the top of my head, but you can make just about anything happen with some scripting. Speaking of which...
4) If you haven't had any programming experience, there'll be a bit of a learning curve. But the more you learn, the easier it becomes to learn more. I was able to pick up VBS in a few weeks, and that was while balancing a busy schedule.
5) Yes, as far as I know. The way SGDK works is really... um... transparent, maybe? You can see all the inner workings pretty easily.
6) Paths are pretty easy.
Dark Elf has a good example of enemies attacking when you're close.
7) Really, it depends on how much you want to do. A very simple game would be made like: making a tileset, creating a map, defining what's solid, putting down paths, creating sprites, setting one sprite to be the player, and setting a starting map. Technically I guess you could skip the solidity stuff, but I wouldn't advise it

.
I think that answers everything. If you've got any more questions, let us know. Hopefully you'll (and we'll) be able to see some of your ideas come to fruition soon.